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152 killer keywords for email subject lines (and 137 crappy ones)

We had a hunch that word choice in email subject lines have a strong effect on response rates. So, we tested 287 keywords across a sample of 2.2bn emails to see which work, and which don’t.

Why? Because President Obama has done more for email marketing than any world leader in the history of mankind. How? By focusing on subject line testing, his digital team optimised their donation campaigns to generate hundreds of millions of dollars online.

Despite Obama’s best efforts, most marketers still view email marketing as the Bluth Company’s Banana Stand of Arrested Development fame: a more boring and less sexy marketing channel than pretty much anything else imaginable.

In case you missed my presentations at MarketingWeekLive last week, you can find out more about our findings after the jump.

We tested a random sample of 95,000 global, English-language campaigns over the last 12 months (for a total of 2.2bn emails), and have isolated 287 popular 'trigger words'.

Then, split by sector, we looked at the correlation between the word’s inclusion in the subject line and its variance above or below the average results for key email metrics (Open Rate, CTR, CTOR, and Unsubscribe).

To ensure outliers aren’t confusing things, we’ve also looked at the first, second and third quartiles to give an indication of data spread, not just nominal long-run means.

Bear in mind, these relationships are correlated, but not necessarily causal. There are simply too many variables in an email campaign to pinpoint exact causation. But, where there’s smoke, there’s fire, and at the very least, this will give you many ideas for what to test.

And now some key findings:

People love free stuff

In related news, water is wet. Specific trigger words have a drastic effect on the response rates of offers. For example, 'Sale' delivers +23.2% opens, but 'Save' only +3.4%.

However, in terms of click throughs, they give +60.7% and -25.2% respectively. When you promote your offers, consider testing those trigger words – it could make or break your campaigns.

And, consider simple quick wins like 'Free Delivery' or even '% off' - in the right context these keywords can drive massive response uplifts. But, if you do nothing but hard sell and offer discounts all the time, your customers will become bored and tune out.

Mix up your offer emails in a series of value-adding campaigns.

Content marketing works when the content isn’t crap

The problem with content marketing is the vast majority of content produced is crap. Too many people have outsourced it to agencies that don’t know enough about their clients’ markets, and focus on the wrong metrics. As a result, consumers have become anesthetised to content.

Conversely if the content is good, people will consume it. So 'News' (+34.8%, +47.7%), 'Bulletin' (+15.8%, +12.7%) and 'Video' (+18.5%, +64.8%) work well.

Simply put, if your content is crap, it won’t work. If your content is good, you’ll get great results. When doing content marketing, make sure you’re in the latter group, not the former, or else you’ll be what we affectionately call in the industry a "spamming %$*!$#".

More frequent emails are better than less frequent

Typical for an email guy to say, right? Full disclosure, when people send out more emails, ESPs make more money, this is true. But, if emails drive response and therefore generate revenue, then what’s the problem?

Personalisation works, if your data is clean

Lots of people have played around with subject line personalisation, with varied results. We found the average opens and clicks decrease (-20.7% opens). But, the spread of the data is massive. The first quartile is -73.1%, but the third quartile is +30.8%. Why is this?

The main thing about personalisation is to ensure you don’t deliver users a disjointed user experience. If the subject line is personalised, but the email content isn’t, guess what? You may gain opens, but have done nothing to drive clicks. It leaves users with a negative experience.

Furthermore, if the subsequent online journey isn’t equally personalised, once again you’ve left your users feeling like you’ve given them the old bait-n-switch.

Most importantly, if your database is old and potentially incorrect, you’re in trouble. Eyeball your data first and pick recent, engaged data for testing personalised content.

There’s always money in the banana stand

This report analysed a vast sample of subject lines. More than anything, it showed there is lots of short-run variance in every keyword sample. What matters is that you test things out to your lists, and never stay standing still. What works one week may not work the next, but if you aren’t trying out new things then you’re treading water and ultimately throwing away money.

Email still delivers the strongest revenue of any digital channel by a country mile (source). In all your tweeting, pinning, and facebooking, are you making sure you dedicate enough time to your email subject lines?

Remember – of Obama’s 30 person digital team, 24 of them were focused on email.

They knew that there’s always money in the banana stand.

To see the full list of keywords (287 common words were tested in addition to the ones above), you can download the report. (Registration required - Ed)

@Steve - great point. I agree that context is everything. However, you'll note that I'm strictly looking at long run correlations and there is indeed a large spread of data, a function of context. With that said, there are clearly some words that across the board perform less strongly than others. I suppose the main point, at the risk of being blunt, is to not be stupid. (And I bet "Free lager" works even better, right?) It's important to note that this is an English-language only sample, so that will control for multilingual variance.

Regardless, I fundamentally believe that subject lines provide context for the entire email - so psychological cues that add to action-taking in the email itself. It puts people in a frame of mind when they're viewing the email content itself - sort of like a prompt to consider undertaking action later on in the web journey.

@kudani - more than happy to discuss your idea and my methodology. Can you suggest a better alternative?

High response rate can be achieved relevancy,offer & incentive in subject line. And remove friction and anxiety from the subject lines. Also,these words would definitely help to get more open rate.

about 5 years ago

Shoplet Promos, Digital Marketing at Shoplet Promos

@Steve You bring up an interesting point but I can't say I completely agree with what you're saying. Context IS everything. Lager coming out on top means nothing for businesses that don't sell lager. Just like the fact that "News" is a stronger subject word than "Report" or "Newsletter" means nothing to someone that is not emailing about news.

When you provide context, you can't ignore data. As always, it's a great tool to catch general trends that the human eye would miss. High-engagement emails most likely have something in common in order to attract clicks and click throughs.

@Fred - yeah, I get asked this a lot. The thing is, spam filters are more intelligent than back in the day, it's not just about content. It's about way more than that - sender reputation, infrastructure, data quality, engagement, etc. So if your brand sends out good emails to quality data, you can get around some of the old rules.

Anyways, the word "sex" (with spaces around each side to control for things like "essex") provide the following:

Mind you, this is only on a sample of 450k emails, so could be some variance issue in there.

@Shoplet - true dat. This report is strictly looking at correlations between keywords and response metrics, but they don't exist in a vacuum. In no way am I saying "Don't use the word report" - what I am saying is, if you use that word, make sure it's relevant to the list, used non-stupidly, and relates to the content of the email (and the rest of the web journey to boot!)

about 5 years ago

Tommy Orme, Copywriter at Canonbury Publishing Ltd

Hey Parry, great article.

Love that stat about 'Sale' delivering +23.2% opens and 'Save' only +3.4%. Looking forward to going over the full report.

Regarding the "no smoke without fire" comment... ice cream sales and murder rates spring to mind, positively correlated in the summertime. Trite example but helps to keep it in mind when considering correlations

about 5 years ago

Phil Manger, Senior Email Marketing Analyst at Future Publishing

Did you find that adding urgency to a subject line increased opens / clicks?

@trudi - for sure. And also, did you know that umbrella sales cause it to rain in London? :) So yeah, these are correlations (try finding causality with so many disparate variables!) but at the same time, the whole purpose of this exercise is to get people to think about their subject lines!

@Phil - so some worked sometimes, and some didn't other times. Great answer, right? Urgency can work but play around with different urgency drivers in the subject lines to see which ones your lists respond to the best.

Surely we are more sophisticated marketers than to think there are any easy solutions - some 'magic' trick.

There are no 'killer' keywords.

As others have said above - context is everything.

Is this a first email to someone who is not a customer, or an email they get every week: an email once in a blue moon with an offer, or an email after they have become a customer. An email asking for feedback. An email linked to a Fecbook cmpaign..a TV campaign,, a viral campaign...

All that context really does matter.

And this is simplistic:
> Simply put, more email drives more response.

Please people, don't take that as an OK to email every day!
Unless you have a marketing strategy that supports that.

As I said; we misunderstood marketing, if we think it is about blindly following magic recipe rules.

@Deri - I'm not sure you've interpreted the blog and the report in the way it is intended. For years email has taken a back seat to other channels - and people split test things like subject lines much less than they should.

This report is intended to get people thinking about subject lines - not a definitive list of which keywords work and which don't. Simplistic, perhaps - but if it gets email marketing back on the agenda for digital marketers then that's the most important thing.

Firstly, for an interesting, thought-provoking post. The fact that it was the latter is clearly shown by some of the above comments. Which brings me onto the 2nd thing you deserve credit for... your patience in responding to some of the comments where people may just have missed the tone of your post :)

And according to Obama's marketing team, their best performing subject line was 'Hey'. Yep, that was it, apparently it brought them in millions of dollars (see http://goo.gl/pypV2 ).

Now if only Obama had gone with 'Free lager & sex'... Mind, then he'd probably have become Prime Minister of Italy instead ;)

My first thought is that Free is a spam word and should be avoided at all costs. Some useful tips in the article, and stats are great for re-inforcing my messages to clients who still think newsletters are the best way of communicating. I agree that context is important, talking to targeted audiences in the right language is key, not just picking key words. Thanks for sharing!

Dennis - the evidence we should take away from the Obama campaign is not:

> according to Obama's marketing team, their best performing subject line was 'Hey'

Because that 3-letter Subject has NO wide-spread usefulness: we canot learn from that word what to put in our own Subject lines!

It's simply what was proven to work, in the situation of the Obama campaign, at the time they were using it.

It probably wouldn't even work as well if Obama himself used it again, in today's situation!

What we CAN learn as marketers and act on to our benefit - is the Process that they used, to find their best Subject lines: that process is something we can all use and will work in all our individual markets and brand situations:

“We did extensive A-B testing not just on the subject lines and the ... The campaign would test multiple drafts and subject lines—often as many as 18 variations—before picking a winner to blast out to tens of millions of subscribers. “When we saw something that really moved the dial, we would adopt it,” says Toby Fallsgraff, the campaign’s e-mail director, who oversaw a staff of 20 writers."

What we can NOT take away is that the word 'Sale' is somehow generically better than 'Save' - even if Parry's data did say it has better click-through.

(and anyway, click-through is one step removed from opening rates and Subject lines really only directly impact the latter, not the former)

@Julie:
Re using 'free', you might be interested in Hubspot's testing of this http://goo.gl/MCwmV that showed that surprisingly, it didn't affect deliverability, although it didn't pull such a good response for them, either (though there could of course be other reasons for that).

@Deri:
Thanks for the comment, but please be assured that if it appeared that I was suggesting that we all adopt 'Hey' as the ideal subject line, it is only because my typing must have been affected by my tongue being firmly in my cheek...

However, if writing more seriously, yes, I agree with you and Obama's team completely (and would suggest that this is the real relevance of the success of 'Hey') in that the key is to test, as what works for one won't for another, or even necessarily for yourself the next time, either!

But, what both Obama's team's results, and Parry's statistical analysis do help us with is firstly to make us realise that sometimes small changes can make a significant difference (for example, we've previously got a 28% higher open rate with the same subject line just by changing its capitalisation away from Title Case). And secondly, they sometimes make us think of changes that maybe we wouldn't have otherwise - like using short, sharp subjects in a personalised context, or maybe how much difference substituting one word for another with a very similar meaning could make.

As for whether or not 'Sale' is somehow generically better than 'save', sure, it doesn't prove it categorically; however, personally, if I had to opt for one over the other in an email subject line tomorrow and couldn't split-test them both, I'm happy to err on the side of Parry's results from 2.2bn emails. And coincidentally (or not) Adestra's stats from 2012 would suggest that it's the best-performing 'discount term', too - see point #22 here http://goo.gl/Qe9Op

Finally (phew), our constant analysis of our own clients' sends shows a very firm and very real correlation not just between subject lines and opens, but with the resultant click-throughs, too. We can always up someone's open rate with a nice, juicy subject line, but if it isn't relevant to its content, the clicks will die.

about 5 years ago

Chuck

@Deri

You make a completely valid point, but I think you're in danger of going round in circles. It's something that's made clear in the article:

Context is extremely important.

What the author's done is present us with some intriguing, 'food for thought' results which encourage us, as marketers, to continually monitor, tweak and split test our subject lines.

That's the takeaway here.

Of course there's no one size fits all. Context is key, but we can still learn from this data. Take some of the ideas, tweak them for your niche and then test, test, test.

about 5 years ago

Tukey

Far better an approximate answer to the right question, which is often vague, than an exact answer to the wrong question, which can always be made precise.' -Tukey

Alchemy Worx launch Touchstone - new Subject Line results predictor and optimisation tool. It will do away with the need to A/B split test. Sign up for the Beta trial now!
http://www.subjectlinegold.com - its free!

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